1. Field of the Invention
This invention relates to a method for evaluating the state of a polysilicon film generated on annealing amorphous silicon, an inspection apparatus for inspecting a polysilicon film formed on annealing amorphous silicon, and a method for preparation of a thin-film transistor provided with a polysilicon film generated on annealing amorphous silicon. This invention also relates to a method for preparation of a thin-film transistor of a bottom gate structure in which a gate electrode is formed between a substrate and a polysilicon film.
2. Description of Related Art
Recently, a thin-film transistor employing a polysilicon film as a channel layer is being put to practical use. If polysilicon is used in the channel layer, electrical field mobility of the thin-film transistor is increased significantly. Thus, if the transistor is used as a driving circuit for e.g., a liquid crystal display, it is possible to realize high definition, high speed and a small size of the display.
Recently, the so-called low-temperature polycrystallization process of heat processing amorphous silicon to form a polysilicon film, using an excimer laser annealing device, has also been developed. By applying the low-temperature polycrystallization process to the production process for the thin-film transistor, it becomes possible to reduce thermal damages to a glass substrate to enable the use of an inexpensive heat-resistant glass substrate of a large surface area.
However, with an excimer laser annealing device, used in the low-temperature polycrystallization process, the grain size of the polysilicon generated is varied significantly because of the unstable output power. So, with the polysilicon film, formed using the excimer laser annealing device, an optimum grain size is not necessarily produced, thus occasionally producing rejects.
Thus, in the annealing employing this sort of the excimer laser annealing device, the state of crystals of the polysilicon film formed on the uppermost surface at a stage of completion of the polycrystallization process for the polysilicon film is checked by 100%-inspection or by random product sampling to check whether or not the product at this state is a reject. Also, the energy information furnished to the polysilicon film is fed back to the excimer laser annealing device to set an optimum laser power.
However, in evaluating a polysilicon film, there is no other method than a sensual method of photographing a surface image using a spectroscopic ellipsometric method or a scanning type electron microscope to view its surface image visually to verify the crystal state, while there lacks a method by non-contact objective check. Moreover, these methods are poor in efficiency as to time and cost and are difficult to use as an in-process method.